1. Field of the Invention
This invention relates to azoalkane mixtures containing unsymmetrical azoalkanes, a process for preparing such mixtures and the use of such mixtures in polymerizing vinyl monomers and curing unsaturated polyester resins.
2. The Problem
Various important properties of azoalkanes (R--N.dbd.N--R'), such as physical state, solubility, volatility, toxicity, thermal stability, and initiator efficiency, are dependent upon the nature of the alkyl groups R and R'. The thermal stability of the unsymmetrical azoalkanes (R.noteq.R') is intermediate between the stabilities of the symmetrical azoalkanes R--N.dbd.N--R and R'--N.dbd.N--R' (P. S. Engel, Chemical Reviews, 80, pp. 101-109 (1980). Therefore, many of the unsymmetrical azoalkanes have unique thermal and initiator properties (depending upon R and R'), which cannot be duplicated by the symmetrical azoalkanes. These unique properties make them commercially desirable as free radical initiators. This is especially true when R and R' are tertiary groups.
Mixtures of an unsymmetrical azoalkane and the corresponding symmetrical azoalkanes have unique thermal and initiator properties that may extend over a wide temperature range depending upon the R and R' groups. For example, when tert-octylamine (1,1,3,3-tetramethylbutylamine) and tertbutylamine are used in the sulfamide reaction, a mixture of di-tert-octyldiazine (which has a ten hour half life (t-1/2) of 107.degree. C.) and tert butyl tert-octyldiazene (which has a 10 hour half life of 137.degree. C.) is obtained; the 10 hour half lives were extrapolated from data in J. W. Timberlake et al 1976 publication in the Journal of Organic Chemistry, vol 41, page 1666. Di-tert-butyldiazene (which has a 10 hour t-1/2 about 160.degree. C. and a b.p. of 109.degree. C.) is removed during the workup. Mixtures of tert-octylamine with tert-amylamine or tert-dodecylamine (Primene 81-R product) give mixtures of azoalkanes with similar half lives as above. Mixtures of initiators of various activities are commonly used in various polymerization processes. Mixtures of peroxides of different half lives are used in the polymerization of styrene, vinyl chloride, and ethylene. Mixtures of azonitriles are also used in polymerizations but these leave toxic residues in the polymers. Mixtures of azoalkanes derived from tert-octylamine and another tertiary alkyl primary amine are particularly useful where a low or intermediate temperature initiator is required to initiate the polymerization at a reasonable rate and a higher temperature initiator is required to finish the reaction. This is especially true in the bulk polymerization of styrene where a high temperature initiator is required to obtain faster conversion to polystyrene. Peroxides cannot be used in this capacity because they degrade polystyrene at the higher temperatures. Azonitriles are too unstable to be used as finishing catalysts. In fact, prior to our invention, no such suitable commercially feasible initiator system existed.
The ratio of the azoalkanes in the mixture and therefore the activity of the azoalkane mixture can be varied by varying the ratio of the amines used in the sulfamide reaction. The activity of the mixture can also be altered by adding one of the pure symmetrical diazenes to the mixture.
Prior to the present invention, mixtures of azoalkanes containing unsymmetrical azoalkanes were not commercially available due to the difficulty involved in preparing the unsymmetrical azoalkanes. Prior to the present invention, unsymmetrical azoalkanes were prepared by a variety of methods which were tedious, had many steps, and were not applicable to all classes of unsymmetrical azoalkanes.
3. Prior Art